I have an old (3 years old, maybe older) Jabra model bluetooth earpiece, and most of the time people don't have any problems hearing me (if the earpiece is on my left ear, not my right) or me hearing them. Although if they can't hear me, I roll up the windows and they hear me just fine (even with the top down).

Note that this is all talking about going between 70-80 mph on the interstate.